Deformation and age of the Red Mountain intrusive system (Urad

Transcription

Deformation and age of the Red Mountain intrusive system
(Urad-Henderson molybdenum deposits), Colorado:
Evidence from paleomagneticand 40 Ar/ 39 Ar data
JOHN W. GEISSMAN Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116
LAWRENCE W. SNEE U.S. Geological Survey, MS 963, Box 25046, Denver Federal Center, Denver, Colorado 80225
GARRETT W. GRAASKAMP Dunn Geoscience Corp., 5 Northway Lane, North, Latham, New York 12110
RICHARD B. GARTEN U.S. Geological Survey, APO New York, New York 09697-7002
ENNIS P. GERAGHTY Stillwater Mining Company, Box 365, Nye, Montana 59061
Colorado Geological Survey,
Earthquake Reference Collection
ABSTRACT
Paleomagnetic and 4()Ar/39Ar age-spectrum data from most stocks of the Red Mountain intrusive system, in the northwest Colorado mineral belt, provide an improved
understanding of the structural and cooling
history of the suite of intrusions host to a
world-class molybdenum deposit. Paleomagnetic data from five stocks at the surface and
eight younger stocks exposed in the subsurface Henderson Mine support field observations (for example, dike and vein orientations,
stock geometries, and distribution of zones of
mineralization) that imply moderate tilting
(15°-25° down to the east-southeast) since
latest Oligocene time after cooling and mineralization. Surface stocks contain magnetizations carried by both magnetite and hematite.
The Red Mountain stock is the youngest surface intrusion and contains mostly normal polarity magnetizations (for example, D = 321°,
I = 59°, a95 = 19°, k = 9, N = 6 samples, site
RM9), whereas older East Knob and Rubble
Rock breccia intrusions contain a nearly antipodal, well-characterized magnetization
(East Knob stock: declination = 161°, inclination = -47°, 095 = 13°, k = 23, average of five
site means). Polarity changed from reverse to
normal during emplacement and cooling of
the Red Mountain intrusions exposed at the
surface. 40Ar/3'Ar age-spectrum data on biotite and orthoclase from the Red Mountain
stock and stocks of the Henderson Mine indicate the reversal to be older than 30 Ma. All
Henderson Mine stocks have normal polarity
magnetizations (Primos stock: D = 333°, I =
51°, 095 = 5°, k = 44, average of six site
means) which, on the basis of 40Ar/39Ar age
spectra from orthoclase and biotite, were
blocked between 28.7 and 27.6 Ma. Magnetite and maghemite are the major carriers of
magnetization in these rocks.
On the basis of an Âr/Âr thermochronologic study of the Red Mountain intrusive
system, thermal activity started at or just before 29.9 + 0.3 Ma and ended at 26.95 ± 0.08
Ma. The age-spectrum data are interpreted to
indicate that the porphyry of Red Mountain,
one of the oldest stocks, was emplaced before
29.9 ± 0.3 Ma (possibly before 30.38 ± 0.09
Ma). Nearby lamprophyre dikes were emplaced at 29.8 ± 0 . 1 Ma; rhyolite dikes intruded at 29.4 ± 0.2 Ma. The Urad and
Seriate stocks intruded after 29.8 Ma but before emplacement of the Vasquez stock at
28.71 ± 0.08 Ma. The system core cooled
below 280 ± 40 °C (the argon closure temperature of biotite) at 27.59 ± 0.03 Ma. The last
period of thermal activity involved pulses of
magnetite-sericite alteration around the Seriate stock between 27.51 ± 0.03 and 26.95 ±
0.08 Ma; this activity did not thermally overprint unaltered parts of the intrusive system.
Tilting of the Red Mountain area is implied
by a comparison between a grand mean (on
the basis of 10 stock means, D = 333°, I = 49°,
«95 = 5°, k = 78) and a mid-Tertiary reference
field. The Red Mountain intrusive system and
host Precambrian rocks probably were deformed along a nearly north-south horizontal
axis in response to northwest-side down,
strike-slip faulting with displacement largely
along the Woods Creek fault zone. Late Tertiary deformation of Precambrian-cored parts
of the Front Range, host to numerous mineral
deposits, was more complicated than simple,
near-vertical uplift of the crust.
Geological Society of America Bulletin, v. 104, p. 1031-1047, 15 figs., 5 tables, August 1992.
1031
INTRODUCTION
Paleomagnetic methods are useful for quantifying crusta! deformation within regions of
greatly varied scale. For batholiths and stocks,
paleomagnetic analysis is one of the few methods by which total deformation may be inferred
(Beck, 1980; Geissman and others, 1980;
Geissman and others, 1982; Shaver and McWilliams, 1987; Faulds and others, 1992). The lack
of conventional reference to the paleohorizontal
datum, however, at the time during which magnetization was acquired has occasionally led to
controversy over the interpretation of paleomagnetic data from large, batholithic terranes
(Beck, 1980; Butler and others, 1991; Marquis
and Irving, 1990; Irving and Thorkelson, 1990).
Other methods may allow estimation of the paleohorizontal datum. Whole-rock and mineral
chemistry, isotope data, and field studies allowed Barnes and others (1986a) and Barnes
and others (1986b) to infer postcrystallization
tilting of Mesozoic plutons in the Klamath
Mountains. Petrologic studies of the Jurassic
Yerington batholith, west-central Nevada, led
Dilles (1987) to corroborate structural (Proffett,
1977) and paleomagnetic (Geissman and others,
1982) investigations that indicated about 70°90° tilting of the intrusions and wall rocks.
Structural setting is important in the exploration for and resource evaluation of mineral deposits in intrusive rocks. Nonetheless, because
of complications resulting from complete or
partial thermal and/or chemical remagnetization, few paleomagnetic studies have focused on
complex, multiple-intrusion and hydrothermally
mineralized systems. Particular concerns are the
time at which magnetization was acquired relative to the time of system intrusion and tempera-
1032
ture changes during intrusion and alteration.
These may be answered by 40 Ar/ 39 Ar agespectrum data on different mineral phases with
different argon closure temperatures.
At the Red Mountain intrusive system, in the
Front Range in Colorado (Fig. 1), late Oligocene stocks host the Urad (surface) and Henderson (underground) porphyry molybdenum deposits (Wallace and others, 1978; Garten and
others, 1988). Several field relationships imply
that after mineralization the Red Mountain system may have been tilted in latest Oligocene and
younger time. These include the asymmetrical
GEISSMAN AND OTHERS
orientation of intrusive contacts, radial dikes,
and veins as well as the asymmetrical distribution of intrusive textures, alteration facies and
ore zones (Wallace and others, 1978; Garten
and others, 1988; Geraghty and others, 1988).
The system intrudes Middle Proterozoic Silver
Plume Granite and, therefore, the paleohorizontal plane at the time of intrusion cannot be referenced. The nearest outcrops of Phanerozoic
strata are 18 km to the northwest in the Fraser
Basin south of Granby (Fig. 1). Differentiating
asymmetries caused by deformation from those
related to intrusion bears on continued explora-
Figure 1. Location of Red Mountain (RM) with respect to the regional tectonic setting of the
northern Rio Grande Rift and associated fault system (after Tweto, 1975, 1979). Inset shows
major faults near Red Mountain; cross section A-A' is shown in Figure 14. Uplift boundaries
defined by the extent of Proterozoic rock (stippled). Fault traces (heavy lines) generalized after
Levering and Goddard (1950), Theobald (1965), Wallace and others (1978), and Theobald and
others (1983).
Geological Society of America Bulletin, August 1992
tion and development of the deeper levels of the
Henderson deposit as well as the Neogene structural setting of the Front Range.
Paleomagnetic and 40 Ar/ 39 Ar age-spectrum
data obtained from most surface and subsurface
intrusions provide an improved understanding
of the structural and cooling history of the intrusive system; many of our observations may be
applicable to other porphyry systems. The paleomagnetic data support geologic evidence for
15°-25° of down to the east-southeast tilting of
the system in latest Oligocene time. Both field
structural and paleomagnetic data have simultaneously documented a paleomagnetically
measureable amount of local tilting of a series of
shallow stocks. The presence of dual polarity
magnetizations indicates that stocks cooled during at least one geomagnetic field polarity reversal (from reverse to normal polarity) before
about 30 Ma and that later thermochemical activity during one or more normal polarity chrons
did not unblock the reverse polarity magnetization. 40 Ar/ 39 Ar age-spectrum data identify the
episodic nature of intrusion and alteration; they
are interpreted to indicate that cooling below
biotite argon-closure temperature of the entire
intrusive system after emplacement of the underground stocks occurred in about 2 x 106
years. Paleomagnetic and 40Ar/39Ar age-spectrum methods used in this study are summarized
in the Appendix.
The 40 Ar/ 39 Ar age-spectrum data presented
below are summarized from an ongoing study
(L. W. Snee and R. B. Garten, 1990, U.S. Geological Survey, unpub. data) on the Red Mountains intrusive system and related Urad-Henderson mineral deposit. All argon data and their
interpretation are included in a forthcoming
paper to focus on the age and thermal history of
plutons and alteration within the intrusive system as well as the origin of the Urad-Henderson
deposit. Because of the importance of highresolution cooling ages for interpretation of the
paleomagnetic results presented here, however,
we include interpreted cooling ages, composite
age-spectrum diagrams, and an interpreted
thermal history derived from these unpublished
argon data. We summarize all existing geochronologic data from previous published and unpublished studies for completeness. The Âr/
39
Ar data are favored in the interpretations
presented below because of higher precision, in
some cases as good as 0.1% or 30,000 years.
GENERAL GEOLOGY
The Red Mountain intrusive system lies
within the 1.4-Ga Silver Plume Granite batholith (Fig. 2). The northeast-striking Berthoud
Pass and north-striking Vasquez Pass faults are
RED MOUNTAIN INTRUSIVE SYSTEM, COLORADO
Figure 2. Generalized surface geology of
Red Mountain; modified from Wallace and
others (1978) and Geraghty and others
(1988). Solid circles are numbered paleomagnetic sampling sites. Medium heavy lines are
dikes and small intrusive bodies. D, U =
down, up.
continuous for several tens of kilometers in the
Front Range in Colorado and pass within 2 km
of the system (Figs. 1 and 2). The intrusive system is bounded by the Woods Creek fault, a
northeast-striking strand of the Berthoud Pass
fault, the Vasquez Pass fault, and a suspected
east-striking fault in the valley of the West Fork
of Clear Creek (Theobald and others, 1983;
Figs. 1 and 2). The specific geology of the Urad
and Henderson deposits and the Red Mountain
area in general has been discussed by Wallace
and others (1978), White and others (1981),
Garten and others (1988), Levering and Goddard (1950), Tweto and Sims (1963), and
Theobald (1965).
The Red Mountain intrusive system is made
up of 15 major stocks and 4 igneous breccia
zones. Of these 19 stocks and breccia zones, 13
were sampled for paleomagnetic study; from
oldest to youngest they are as follows: breccia of
East Knob, breccia of Rubble Rock, Red Moun-
Silver Plume Granite ;
Silver Plume Granite
Figure 3. Location of
numbered subsurface paleomagnetic sampling sites
(solid circles) with respect
to stocks in the Henderson Mine, projected
to the 7,500-ft (2,330-m)
level. Geology modified
from Wallace and others
(1978). Lettered stocks
are as follows: H, Henderson; P, Primos; S,
Seriate; B, Berthoud; A,
Arapaho; R, Ruby; N,
Nystrom. Dashed lines refer to fact that the Arapaho and Berthoud stocks
are not exposed at the
7,500-ft level. East/north
coordinates are specific to
the Henderson Mine.
1033
1034
GEISSMAN AND OTHERS
tain border, porphyry of Red Mountain, igneofragmental breccia (all exposed at the surface),
Urad, Berthoud, Henderson, Primes, Arapaho,
Seriate, Ruby, and Nystrom (Fig. 3). All stocks
and fragments in intrusion breccia are similar in
chemistry and mineralogy and consist of quartz,
alkali feldspar, albitic plagioclase, and minor
biotite. The surface rocks are generally porphyritic and locally exhibit clastic and/or fragmental textures consistent with subvolcanic emplacement. Sample sites on the surface of Red
Mountain lie within a general zone of phyllitic
quartz-sericite-pyrite alteration and a more distant outer zone of argillic alteration (MacKenzie,
1970). These rocks contain abundant hematite,
presumably formed during alteration of pyrite,
biotite, and/or magnetite. The paleomagnetic
data discussed below suggest, but do not conclusively prove, that oxidation occurred during hydrothermal activity, not during recent, nearsurface weathering. At most subsurface sites
(Henderson Mine, -1.3 km below the surface of
Red Mountain), intrusions are porphyriticaplitic to equigranular in texture. With respect
to our collection for paleomagnetic investigation, the type and intensity of hydrothermal alteration and degree of molybdenum mineralization vary among individual samples from a site
and among specimens from a given sample.
Typically, secondary sericite and kaolinite
(Walker, 1984) partially replace feldspar at
many sites.
Field relations suggest that the intrusive system was not emplaced in its present, off-vertical
orientation. First, mineralized rocks and highsilica alteration halos delineated underground
are best developed southeast of the apical parts
of mineralizing stocks (Garten and others,
1988). Second, primary igneous textures within
individual stocks (Shannon and others, 1982)
and geometries of stock contacts imply that
structurally higher levels of stocks presently are
at lower elevations on their southeast sides than
on their northwest sides. Third, radial dikes exposed on the surface and radial vein sets exposed
underground in the Henderson Mine are markedly asymmetric in orientation. The dike and vein
orientations imply either a nonvertical principal
stress direction during dike emplacement or tilting (rotation about a near-horizontal axis) of the
system after intrusion, hydrothermal alteration,
and mineralization (Geraghty and others, 1988).
PALEOMAGNETIC
RESULTS
Red Mountain Surface Stocks
Magnetizations from samples of the five intrusions (nine sites) on Red Mountain are reasonably well defined and suggest acquisition over a
2 ml
NRM
RMT1D13,
East Knob
RMT1G2a,
East Knob
660 \ RMT3D1a,
East Knob
Figure 4. Orthogonal progressive demagnetization diagrams (Zijderveld, 1967) of representative response by Red Mountain surface rocks. The endpoint of the magnetization vector is
simultaneously projected onto two orthogonal planes (horizontal: E-W/N-S plane, solid symbols; vertical, E-W or N-S/U-D plane, open symbols). Peak demagnetizing inductions in
millitesla or temperatures in degrees Celsius (°C) are given adjacent to data points on the
vertical projections. Data are plotted in geographic coordinates. Specimen identifier is given
before a brief description of the rock. Magnetization directions isolated in progressive demagnetization and determined using principal component analysis are as follows: (A) 159°/-34°
(declination/inclination), maximum angular deviation (MAD) = 3.4°, 300-560 °C; (B)
137V-440, MAD = 3.8°, 25-80 mT; (C) 100°/-59°, MAD = 4.0°, 23-80 mT; (D) 143°/-48°,
640-660 °C; (E) 134°/-52°, MAD = 8.0°, 300-627 °C.
time period of reverse and then normal polarity.
Median destructive inductions (MDI) in alternating field (AF) demagnetization (the peak induction required to reduce the natural remanent
magnetization [NRM] intensity to half the original value) commonly exceed 15 millitesla (mT).
For MDI's exceeding 15 mT, the NRM is carried predominantly by fine (pseudo-single- or
single-domain) magnetite, maghemite, and (or)
hematite. Magnetizations carried by hematite
(with unblocking temperatures greater than
-580 °C and MDI's > 100 mT) are usually similar in direction to those residing in magnetite,
unblocked at lower temperatures.
The two oldest intrusions are breccias (East
Knob and Rubble Rock); they contain magnetizations of southeast declination and moderate
upward (negative) inclination (Figs. 4 and 5A).
We interpret this magnetization as a reversepolarity-thermoremanent magnetization (TRM)
characteristic of the rocks. In single-component
magnetizations, the reverse-polarity characteris-
N
Figure 5. Paleomagnetic
data from the Red Mountain intrusive system.
(A) Stocks exposed at
the surface of Red
Mountain (Table 1).
(B) Henderson Mine
stocks (Table 2). Sitemean directions of magnetizations isolated in
progressive demagnetization with associated
projected cones of 95%
confidence. Equal area
projections with closed
(open) symbols representing lower (upper) hemisphere projections.
tic magnetization usually resides in magnetite
(see, Figs. 4A and 4B), but commonly much of
the NRM is carried in hematite, as revealed
in thermal demagnetization. Although short
(-10,000 yr) reversed-polarity subchrons occurred during the Brunhes normal polarity
chron (ca. 730 Ka to present) (Champion and
others, 1987), magnetizations of reverse polarity
carried by hematite were probably not acquired
during recent weathering. The relationship is
more complicated where behavior is multicomponent, such as site 3 in porphyry at East Knob.
The reverse-polarity remanence in site 3 samples
is carried by both hematite (see Fig. 4D) and
magnetite (see Fig. 4C). When magnetite carries
the reverse-polarity magnetization, higher unblocking temperature magnetizations residing in
hematite are of positive inclination yet are
dispersed in declination.
The border and porphyry phases of the Red
Mountain stock and the igneo-fragmental breccia cut the East Knob and Rubble Rock breccia
stocks. These younger intrusions contain magnetizations of mixed polarity, but the magnetizations are predominantly of northwest declination and moderate positive inclination (normal
polarity). The remanence is carried by magnetite
and hematite. Using the paleomagnetic data
summarized in Table 1, we infer that intrusion
of stocks exposed on Red Mountain spanned at
least one field reversal.
Magnetizations carried by hematite in the
Red Mountain intrusions may have been acquired over a prolonged period of oxidation that
spanned several polarity chrons. In some rocks,
more than 80% of the NRM is carried by hematite. Because the directions of magnetizations
carried by hematite generally parallel those residing in magnetite, we suggest that the magneti-
1035
zations in hematite record an ambient field; the
magnetizations could have been acquired during
the same polarity chron as when a TRM was
blocked in magnetite. We assume that the use of
magnetizations residing in hematite, as well as
those in magnetite, is reasonable for structural
interpretations.
varies considerably (Fig. 5B). Probable contributors to the within-site dispersion of directions
are discussed below. In specimens with multicomponent magnetizations, the first-removed
component usually has a more northward declination and steeper positive inclination (such as
specimen HEN19Bla, Fig. 6E). In total, NRM
directions are somewhat dispersed, but a wellHenderson Mine Stocks
defined, reverse polarity secondary magnetizaAll eight intrusions (30 sites) sampled in the tion was not defined for any site or individual
Henderson Mine are characterized by a normal
intrusion. Unlike the surface rocks, magnetite
polarity magnetization, in contrast to the surface and possibly maghemite are the only magnetizastocks which are principally of reverse polarity tion carriers, because the intensity of magnetiza(Table 2 and Table 3). The magnetization iso- tions remaining above ~580 °C never exceeded
lated in all intrusions has north-northwest decli- a few percent of the original NRM intensity.
nation and moderate positive inclination (Fig. MDI's for most rocks containing magnetite as
6), although the amount of within-site dispersion
the principal magnetic phase are < 15 mT, sug-
TABLE I PALEOMAGNETIC DATA FROM SURFACE STOCKS OF RED MOUNTAIN
Slock
East Knob
Easl Knob
Easl Knob
East Knob
Easl Knob
Mean
Rubble Rock,
breccia
Red Mountain
border
Red Mountain.
porphyry
Iffneo-fragmen.
breccia
Suenumber
In situt
R
°95
It"
n/n0/Ntt
16
5
12
II
6
23
9/9/7
7/11/7
8/10/7
12/12/8
8/10/7
5 siles
decl.
incl
I
2
3
4
7
149
209
153
175
163
168
-44
-48
-44
-46
-40
-46
12
20
14
12
20
13
6
161
-47
14
21
5/6/5
5
155
-36
10
22
8/10/7
9
321
59
19
9
6/7/6
8
162
-52
23
8
5/6/3
•All sites arc shown in Figure 2.
fy/r silu declination (decl.), in decrees cast of north; inclination (incl.), positive downward of the site or stuck mean.
^Semi-angle of the cone of 95% confidence around the calculated mean direction.
"Best estimate of the precision of the distribution of the n magnetization vectors (Fisher, 1953)
TtThc ratio of progressively demagnetized specimens used for statistical calculations (n) to the total analyzed <n 0 ) from tbe number of independent samples
collected (N).
1036
GEISSMAN AND OTHERS
phases that carry geologically important magnetizations may limit the age of magnetization acquisition relative to subsolidus cooling, alteration, and later uplift and local deformation.
hematite, and goethite (Fig. 7). Textural relations suggest that hematite and goethite have
replaced pyrite and magnetite. Pyrite precipitated during hydrothermal mineralization and
alteration; magnetite is of magmatic origin. Intensities of the NRM for the Red Mountain surface intrusions are low; mean values for each
intrusion range from 140 to 2 milliamperes/
meter (mA/m). Such low NRM intensities attest to a very low concentration of magnetic
phases, especially of magnetite.
Isothermal remanent magnetization (IRM)
acquisition curves usually increase gradually in
magnetization to as much as 1.2 T, but never
reach saturation. The lack of abrupt increase in
IRM in inductions less than 0.3 T implies that
high coercivity phases predominate in the surface intrusions. Where enough magnetite could
be separated from these surface rocks, poorly
defined saturation magnetization versus temperature curves indicate Curie temperatures between 550 and 585 °C, indicative of low-Ti
magnetite as the saturated phase.
The magnetic properties of samples from individual stocks, as well as from individual sites,
are varied which leads to some between-site variations in dispersion of magnetization data. As
mentioned above, demagnetization results from
site RMT3 (Fig. 8) in the East Knob stock exemplify such variations. In thermal demagnetization, specimens from samples G and I exhibit
magnetization unblocking temperatures below
580 °C; those of specimens from samples D and
F are above 650 °C. In AF demagnetization,
behavior of duplicate specimens indicated dominance by magnetite (samples G and I) and
hematite (samples D and F).
Red Mountain Surface Stocks
Henderson Mine Stocks
Sample response to progressive demagnetization, rock magnetic tests, and petrographic examination all indicate that the principal magnetic phases in the surface intrusions are magnetite,
Magnetization carriers in the Henderson Mine
stocks are magnetite and probably maghemite.
IRM acquisition curves show nearly complete
saturation by about 0.3 T which indicates dominance by moderate- to low-coercivity cubic
phases. Hematite was not an equilibrium mineral phase during molybdenite mineralization
(Drabek, 1982; Garten and others, 1988), and
subsequent oxidation of magnetite and pyrite to
hematite has been fairly minor in the underground stocks. NRM intensities for the underground rocks are also low; mean values for each
intrusion range from 3.5 to 40 mA/m. Again,
such low NRM intensities imply only small
amounts of cubic magnetic phases. If fine (magnetically single domain) grains carry the magnetization, then a TMR of 5 mA/m could reside
in grains whose volume fraction is less than -2.6
x 10-6(Sugiura, 1979).
Magnetite grains are predominantly inclu-
TABLE 2. PALEOMAGNETIC DATA FROM HENDERSON MINE SUBSURFACE STOCKS
OF RED MOUNTAIN
Stock
Site'
In siiu*
decl.
Urad
1
4
7
17
21
23
326
316
322
292
2
343
328
69
57
25
61
45
56
54
20
2
3
5
16
24
348
326
323
313
310
332
320
6
15
25
26
27
28
20
20
IS
14
4/6/4
6/8/5
2/4/3
4/5/5
3/5/4
4/5/4
6 sites
57
48
51
40
37
46
45
19
14
19
8
12
8
7
13
12
10
28
45
57
82
4/4/4
8/9/5
5/6/4
10/10/5
3/5/3
5/6/5
5 sites
342
332
311
327
339
352
333
39
59
49
43
59
54
51
11
9
21
15
12
22
8.6
26
50
14
14
29
7
44.1
6/8/4
5/6/5
3/5/4
5/6/6
5/5/5
6/6/6
6 sites
19
8
9
10
11
12
13
14
348
323
331
328
335
334
328
299
327
54
53
44
46
46
33
29
59
48
10
31
12
16
II
14
26
15
8
34
6
20
12
22
13
6
14
39
5/7/5
4/5/4
6/8/7
6/7/6
8/10/7
7/7/6
5/6/4
6/6/6
7 sites
18
22
29
30
335
332
331
328
330
44
38
51
54
47
23
12
12
9
8.6
6
24
26
44
88.6
6/6/6
6/6/6
5/5/5
6/7/6
3 sites
Mean
Primos
Mean
Arapaho
Seriate
Mean
Ruby
Nystrom
n/n0/N
41
32
10
13
22
15
Mean
Berthoud
Henderson
*
"95
incl.
Mean
9
10
t
*AII sites are shown in Figure 3.
*The number of samples 'was insufficient to calculate statistical parameters.
gesting that multidomain grains carry much of
the NRM.
MAGNETIC MINERALOGY AND
ROCK MAGNETISM
The history of the Red Mountain intrusive
system includes a complex series of igneous as
well as hydrothermal mineralization and alteration events (White and others, 1981; Garten and
others, 1988). Observations on the magnetic
TABLE 3. SUMMARY OF PALEOMAGNETIC DATA. RED MOUNTAIN INTRUSIVE SYSTEM.
AND EXPECTED FIELD DIRECTIONS
Mean
Pole position
Determination
N/N0
Decl..
Incl.
a9j
k
Stocks
Sites
10/13
34/39
333
332
49
49
5.0
4.3
31.2
14
355
58
86
105
351
57
83
77.5
Lat.
Long.
66
65
149
151
K*
ag5*
22.9
5.6
5.0
154
144
3
148
13
4
62.5
S§
1.7
12.5
Expected directions
Irving and Irving (1982)
(30 Ma)
Dichl and others 1 1983)
(22-38 Ma)
0.3
38.8
•Best estimate of the precision of the N site virtual geomagnetic pole positions.
'Semi-angle of the cone of 95%-contidence defining the location of the true mean pole position at the 95^-confidence level.
Ângular standard deviation of the vinual geomagnetic poles.
Figure 6. Orthogonal progressive
demagnetization diagrams (Zijderveld, 1967) of representative response by Henderson Mine rocks.
The endpoint of the magnetization
vector is simultaneously projected
onto two orthogonal planes (horizontal: E-W/N-S plane, solid symbols; vertical, E-W or N-S/U-D
plane, open symbols). Peak demagnetizing inductions (in mT) or
temperatures (°C) are given adjacent to data points on the vertical
projections. Data are plotted in
geographic coordinates. Specimen
identifier is given before a brief description of the rock. Magnetizations isolated in progressive demagnetization and determined using
principal component analysis are as
follows: (A)334°/50°,MAD = 7.7°,
42-mT origin. (B) A comparison of
response to thermal and AF demagnetization; thermal: 344°/57°,
MAD = 2.3°, 300-500 °C, AF; high
median destructive inductions of
samples from this site preclude isolation of a well-defined component
of magnetization. (C) 330°/38°,
MAD = 3.5°, 25-100 mT. (D)
338°/39°, MAD = 15.9°, 18-95
mT. (E) 323°/42°, MAD = 2.2°,
25-100 mT. (F) 330°/58°, MAD =
1037
HEN2E1b,
Hend.
porphyry
500
HEN9C2b,
Ser. border
- -1 mA/m
8
2
28
NRM[E10C$300
2 mTQ - -4mA/m
NRM
Dn
•'NRM
6C2a M^f
- -0.05 mA/m
80^
NRM
Dn
0NRM
10 mA/m
0.5 mA/m- -
N
HEN19B1a
\
Z porphyry
ry\
50
X
porphyry
100
HEN9C2b\
1.8°, 8-50 mT.
- -100 m A / m
Dn
Dn
E
%2mT
NRM
2 mT
sions in feldspar crystals and are usually euhedral, unaltered, and homogeneous. We found
little petrographic evidence for hydrothermally
introduced magnetite that may contain chemical
remanent magnetizations (CRM) acquired at
subsolidus temperatures. All underground sites
were outside the zone of magnetite-topaz veins
described by Wallace and others (1978) and
Seedorf(1987).
Fine-grained maghemite in some of the
Henderson Mine rocks is inferred from measurements of thermal demagnetization and Curie
temperature (Fig. 9). Specimens from a total of
38 samples were thermally demagnetized as part
of pilot demagnetization. Of these, 25 had linear
trajectories in demagnetization. Response was
characterized by distributed unblocking temperatures with complete unblocking below 580 °C.
On the basis of monitoring low-induction susceptibility between heating steps, no changes in
L
the magnetic mineralogy with temperature were
observed. The other 13 specimens, containing
quartz-sericite-pyrite veins, behaved erratically
between each heating step, especially at temperatures >350 °C. At temperatures greater than
-300 °C, pyrite oxidizes to magnetite and (or)
hematite under atmospheric conditions, hampering thermal demagnetization of pyrite-bearing
rocks (Schwarz and Laverdure, 1982; Geissman
and others, 1983). In most of the 13 specimens,
low-induction susceptibilities increased several
orders of magnitude when the specimens were
heated above 300 °C.
In 10 of the 25 specimens for which demagnetization trajectories were well defined, the
magnetization was dominated by titanium-poor
magnetite. More than 99% of the magnetization
was unblocked by 580 °C; in some specimens
the range of unblocking temperatures is less than
a few tens of degrees. Curie temperatures be-
tween 550 °C and 580 °C are typical of these
rocks, and the heating and cooling curves are
generally reversible. For the other 15 specimens,
>90% of their magnetization was unblocked
over a narrow yet low temperature range between 300 °C and 390 °C. Unlike rocks for
which magnetization resided largely in magnetite, these samples had MDI's exceeding 50 mT.
Results from site HENS in porphyry of the
Henderson stock illustrate this behavior (Fig.
10). In Curie temperature analysis, the saturation magnetization of rocks that have low unblocking temperatures usually decreases upon
cooling (Fig. 9), which probably reflects the inversion of a metastable maghemite phase to
hematite and Curie temperatures of about 600
°C. The low unblocking temperatures of these
rocks also may represent the inversion of maghemite to hematite, a temperature- and timedependent process (Reefer and Shive, 1980;
S s -
S
« o
C (1 «•= a i« -3 £
a
S 2
01
*•
.s?
a s
3 3 Jj
CQ,
5 «
1-5.4)
"•* .5 •=
t.
J * s8
|;
"
;;|
v.
I,
a
a3 aa
8
I ^- o
fa
o -o i
I
3 "Hz S I ' S
BS g 2
. B g> -__ ^ ^ ,0
•s & 31 Q s
.III
fa e o
••a s- SB
1038
1039
HEN 5A3a, Porphyry ,
Henderson Mine
o
CTJ
N
D)
(0
HEN20D13, Porphyry,
Bethoud
100
200
300
400
500
600
700
Temperature, °C
Figure 9. Saturation magnetization versus
temperature heating and cooling curves for
magnetic separates from Henderson Mine
rocks (samples HEN5A3a and HEN20Dla).
Experiments performed in air with an induction of -0.18 T and heating/cooling rates of
10 °C/min.
Ozdemir and Banerjee, 1984). Keefer and Shive
(1980) reported inversion temperatures of 500
°C for pure maghemite and 360 °C for maghemite of oxidation parameter z = 0.5. Ozdemir and Banerjee (1984) found synthetic,
single-domain maghemite to remain stable
below -510 °C and probably to higher temperatures. The discrepancy between experimental
and observed inversion temperatures may lie in
the fact that Ozdemir and Banerjee (1984)
heated samples at a rate of 10-20 °C/min. In
thermal demagnetization, samples are heated
at a fixed temperature for at least 30 min. To account for the observed high MDI's in maghemite, the magnetization must be contained
largely in grains less than a few microns in diameter (Dankers, 1979). Coarse, multidomain
magnetite is present in small quantities at these
sites.
The low unblocking-temperature magnetization could not reside in high-Ti titanomaghemite
grains for which Curie temperatures are <400
°C, because all magnetite in the Henderson system contains < 1.0 wt% TiO2 (Seedorf, 1987). If
samples are heated as Curie and laboratory unblocking temperatures are measured, then saturation magnetization and NRM intensity do not
increase. Single-domain, low-Ti magnetite
grains near the superparamagnetic threshold
have low unblocking temperatures (Dunlop,
1973), but Curie temperatures >600 °C would
not be expected. Lastly, fine-grained pyrrhotite
is also not likely to carry the low unblocking
temperature magnetization. This phase is spatially restricted within the Henderson system,
and these regions were not sampled in the present investigation.
We conclude that low-Ti maghemite carries
the low unblocking temperature and high-MDI
magnetization in some of the Henderson stocks,
and that the maghemite formed during lowtemperature oxidation of fine magnetite at
temperatures <400 °C during alteration and
thermal decay of the hydrothermal system. The
presence of maghemite, although pervasive on a
site level, is not specifically related to an individual pluton. Rather, it appears related to the
intensity of quartz-sericite-pyrite alteration.
Is the maghemite-dominated magnetization in
these rocks an accurate recorder of the field at
the time of acquisition? Experiments producing
fine-grained, low-Ti maghemite from singledomain magnetite particles (Johnson and Merrill, 1974; Heider and Dunlop, 1987) have
shown that the initial remanence carried by
magnetite is preserved in single-phase oxidation
to a cation-deficient spinel, regardless of the direction of the field during oxidation. If realistic
for natural conditions, the experimental data
imply that an original TRM acquired by magnetite during blocking below 580 °C could still be
retained in the most extreme conditions of pervasive maghemitization of Henderson stocks.
40
O Thermal
1.0
D AF
.8
.4
.2
20
40
2
60
3
80
4
100
5
mT
6
7 x100 C
Treatment
Figure 10. Normalized intensity (J/J0) decay curves for specimens from porphyry samples
from the Henderson Mine, site HENS. Open squares indicate results for AF demagnetization,
°pen circles for thermal treatment. Capital letters refer to discrete samples from site HENS.
Ar/39Ar RESULTS
Fission-track and K/Ar isotopic dates from
earlier studies (Table 4; Naeser and others,
1973; Shannon, 1982) poorly define the age of
the Red Mountain intrusive system between
23.8 ± 2.3 and 34.4 ± 1.7 Ma. This wide range
in dates possibly reflects thermal complexities
that resulted from the emplacement of many
closely spaced intrusions, the effect of subsequent alteration of dated phases, and subsequent
cooling of dated phases through corresponding
isotopic closure temperatures. In contrast, the
"OAr/Âr age-spectrum dates (Table 4; L. W.
Snee and R. B. Garten, U.S. Geological Survey,
unpub. data) range between 26.95 ± 0.08 and
29.9 ± 0.3 Ma; all 40Ar/39Ar dates are reasonable when compared to geologic relationships.
These dates were obtained on biotite, orthoclase,
and muscovite; isotopic closure temperatures are
about 280 ± 40, 350 ± 50, and 325 ± 25 °C,
respectively, assuming moderately high (>25
°C/m.y.) rates of cooling (Snee, 1982; McDougall and Harrison, 1988; Snee and others, 1988).
GEISSMAN AND OTHERS
1040
TABLE 4. SUMMARY OF K-Ar ISOTOPIC. Âr/Âr ISOTOPIC AGE SPECTRUM
AND FISSION-TRACK (F-T) AGE DATA FROM RED MOUNTAIN INTRUSIVE SYSTEM
Stock
Number
Age (Ma)
Mineral dated laboratory
Character of
age spectrum*
V ear
Method
Red Mountain Surface Slocks
9
Easl Knob.
porphyry
East Knob,
porphyry
Square quartz
Square quartz
Radial rhy.
Red Mtn..
porphyry
Red Mtn..
porphyry •
Red Mtn..
porphyry
Lamprophyre
29.81 t 0.10
10
Rhyolite dike
29 4 t 0.2
Orthoclase.
4fl
Ar/ 39 Ar
4
Urad Mine.
porphyry
Urad Mine.
porphyry
Urad Mine,
porphyry
Urad Mine.
porphyry
Urad Mine.
porphyry
Urad Mine.
porphyry
Urad Mine.
porphyry
Henderson,
pegmalite
Henderson.
pegmatite
Seriate,
pegmatite
Seriate,
pegmatite
Senate
33.7 t 2.5
Biotile. K-Ar
5
28.5 t 1.2
Orthoclase, K-Ar
5
26.2 t 2.5
Zircon. F-T
I
23.8 t 2.3
Zircon. F-T
1
28.0 ±
Orthoclase.
ÂrX-Âr
Biolite,
*°Ar/39Ar
Biolile. "ArP'Ar
4
1
2
3
4
5
6
7
8
28.5 * 2.2
Zircon. F-T
I
27.2 » 2.6
Zircon. F-T
1
26.9
28.5
34.4
28.7
r 0.9
t 2.2
t 1.7
f 2.9
Sencite. K-Ar
Zircon. F-T
Zircon, F-T
Zircon, F-T
2
1
1
I
28.
t 3
Orihoclase, K-Ar
3
29.9 t 0.3
Onhoclase.
«Ar/»Ar
Biotile. 40 Ar/ 39 Ar4
4
Disturbed,
preferred age
Dis urbed.
preferred age
Disturbed,
excess argon
Henderson Mi ne Stocks
II
12
13
I
i
i
£1
E
<.
l4
15
16
17
(
18
i
!1;
"
J :
20
/;
t:
;-j
i; j
fi
2'
22
2i
.09
28.6 t 0.3
28.3 t 1.4
4
4
Plateau
27.6 tO.l
Alt. onhoclase.
"Âr/Âr
Biolite, '"WÂr
4
Plateau
28.2 t 0.2
Onhoclase.
4
Plateau
Ar
4
Plateau
27.6 ± 0.2
Biotite, Âr/Âr
4
Plateau
28.71 t 0.08
Onhoclase,
•"Ar/3»Ar
Bi«,te.'1<W9Ar
4
4
Disturbed,
preferred age
Plateau
Muscovite,
«Ar/3'Ar
Muscovite,
*>/Ar/ 39 Ar
Biotite, *Ar/Âr
Orthoclase.
40
Ar/39Ar
Orthoclase,
*>Ar/39Ar
Biome, 40Ar/39Ar
4
Plateau
.2
28.0 t 0.2
27.1 t 0.2
5''
t-'; *
28
B '
; '
29
so
f '
31
Ule border
28.4 i 0.3
?
)- '
32
Ute border
27.6 ± 0.2
24
i; 'i
*;
25
fi
k.
V
:•
27
4
Disturbed,
preferred age
Disturbed,
preferred age
Plateau, small
sample
Excess Ar, loss
to 28.1 Ma
Plateau
Onhoclase,
«Ar/-"Ar
Biotile, "Ar/Âr
28.1 i-
Seriate,
granite por.
Senate,
pegmatite
Vasquez,
pegmatite
Vasquez,
pegmatite
Seriate, mag.sericite alt.
Seriate, mag.sericite all.
Vasquez, por.
Vasquez, por.
I \
4
27.6 t 0.2
27.6 i 0.3
26.95 1 0.08
27.51 10.03
27.6 t O.I
27.83 ±0.10
Biolite. *Ar/
39
4
4
Plateau
4
4
Plateau
Plateau
4
Plateau
4
Plateau
All K-Ar isotopic age determinations have been recalculated using revised decay constants. Sample numbers refer to those in Figure 15. Laboratory, year: I. U.S.
Geological Survey (Naescr and others, 1973); 2. Oeochron Labs, summarized in Shannon (1982); 3. U.S. Geological Survey, summarized in Shannon (1982); 4. U.S.
Geological Survey (1990. L W. Snee and R. B. Garten, unpub. data); 5. Geochron Labs, summarized in Shannon (1982).
•Plateaus are calculated according u> the method described in Snee and others (1988); "disturbed" refers to an age spectrum that does not exhibit a "plateau"; a
"preferred age" is calculated for a disturbed spectrum by weight-averaging the apparent ages of temperature steps lhal are statistically identical within three standard
deviations of the weighted average.
In most cases, the age spectra are well defined; in
a few (best exhibited in Figs. 11A and 11B),
excess argon or argon loss are recorded in initial
release of 39ArK.
On the basis of the 40 Ar/ 39 Ar age spectra
(Fig. 11), the emplacement and cooling history
of the Red Mountain intrusive system, summarized in Table 5, is as follows:
(1) We interpret the age spectrum for Orthoclase from the porphyry of Red Mountain (Fig.
11 A) as an indication that the intrusion cooled
below -350 °C (orthoclase closure temperature) following its emplacement before —29.9 ±
0.3 Ma (a "preferred" age based on an average
of last 60% of the age spectrum) and possibly
before 30.38 ± 0.09 Ma (apparent age of the last
temperature step). The age spectrum is disturbed
and exhibits minor excess argon in the lower
temperature steps and evidence for a thermal
disturbance at ~26.9 Ma.
(2) Lamprophyre dikes were emplaced at
-29.8 ± 0.1 Ma and rhyolite dikes were em-
placed at about 29.4 ± 0.2 Ma. These emplacement ages are based on data for biotite from a
lamprophyre dike (age spectrum not shown in
Fig. 11) and orthoclase from the rhyolite dike
(Fig. 11 A).
(3) The Urad and Seriate stocks were emplaced before 28.71 ± 0.08 Ma, which is the
interpreted age of emplacement of the Vasquez
stock. The Ute stock was emplaced between
28.7 and 28.4 Ma. These interpretations are derived from five orthoclase and one biotite age
spectra shown in Figure 1 IB.
(4) The central part, or core, of the intrusive
system cooled below 280 °C at 27.59 ± 0.03
Ma. Age spectra for six biotites from the
Henderson, Ute, Vasquez, and Seriate stocks
within the core of the system (Fig. 11C) are
statistically indistinguishable in age and represent the time during which the system cooled
below the argon closure temperature of biotite
(-280 ± 40 °C).
(5) The magnetite-sericite alteration zone
around the Seriate stock formed between 27.51
± 0.03 and 26.95 ± 0.08 Ma based on
'"'Ar/^'Ar age spectra for three muscovites (Fig.
1 ID) crystallized during this event. One of these
muscovites is statistically older than the two
younger muscovites; this suggests that magnetite-sericite alteration affected different parts of
the alteration zone at different times. An altered
orthoclase from the Seriate stock has a 40 Ar/
39
Ar date (age spectrum not shown in Fig. 11)
of 27.13 ± 0.17 Ma and was probably affected
during the magnetite-sericite alteration event. In
general, however, the lack of thermal overprint
on unaltered parts of the Red Mountain intrusive system indicates that the thermal effect from
the magnetite-sericite alteration was limited.
The interpreted 40Ar/39Ar age-spectrum data
are consistent with observed cross-cutting relations and define the emplacement, cooling, and
alteration history of the system (Fig. 12).
DISCUSSION
Overall, paleomagnetic data from stocks of
the Red Mountain intrusive system are well
grouped on both between-site and betweenstock levels. East Knob and Rubble Rock breccia stocks on the surface of Red Mountain
contain characteristic magnetizations of reverse
polarity. All younger surface stocks contain
magnetizations of mixed polarity. Henderson
Mine stocks contain only normal polarity magnetizations antipodal, at a 95%-confidence level
(McFadden and Lowes, 1981), to those of reverse polarity. Magnetizations defined in progressive demagnetization do not cluster about
present-day or time-averaged Quaternary field
directions. We suggest that the paleomagnetic
data adequately represent a late Oligocene field
1041
40
Figure 11. Composite Âr/Âr age-spectrum diagram for orthoclase, muscovite, and biotite from the Red Mountain intrusive system.
(One sigma analytical errors for data for most statistically significant
temperature steps are between 0.06 and 0.10 Ma.) (A) Age-spectrum
for orthoclase from porphyry of Red Mountain and potassium feldspar from a rhyolite dike emplaced into Precambrian host rocks.
(B) Six (five orthoclase, one biotite) age spectra for minerals from the
Urad, Seriate, Henderson, Ute, and Vasquez stocks. (C) Six age spectra for biotites from Henderson, Ute, Vasquez, and Seriate stocks
within the central part of the Red Mountain intrusive system. (D)
Three age spectra for muscovite from the magnetite-sericite alteration
zone around the Seriate stock.
Porphyry of Red Mountain
30
Rhyolite dike
20
40
Urad, Seriate, Henderson, Ute,
and Vasquez stocks
CO
5
30
c20
£40
ID
a
#30
20
40
B
System core, cooling below 280°C
Magnetite-sericite alteration
30
20
100
50
Percent "ArK released
Red Mountain
and that the results may be used to assess structural deformation of the intrusive system and
host Precambrian rocks. Before discussion of the
structural history, we address the dispersion of
paleomagnetic data.
Dispersion of Magnetization Data
Potential contributions to within-site dispersion of data from the intrusive system include
sampling and data reduction errors; irregular,
prolonged thermal histories of parts of stocks;
and acquisition of CRM's in hematite and
maghemite during alteration of primary magnetic or precipitation of secondary magnetic
phases.
For both surface and subsurface sites, withinsite dispersion is affected by occasional inaccuracy in sample orientation and/or collection of
in-situ material. Red Mountain is rugged, steep,
and largely covered with talus. Although we attempted to collect samples broadly over undisturbed outcrops, parts of outcrops may be
slumped. In subsurface, the fragmentary rocks
and mirior, local, magnetic field disturbances,
for which we could not fully account, may have
increased dispersion. As much as 5° of angular
dispersion may be due to errors in sampling. We
were not able to apply principal components
analysis to results from the entire collection because of an unfortunate accident destroying all
records of most demagnetization data after vector subtraction was applied. Where possible,
comparison of results of both methods yields
vector differences as large as 5°. Differences are
not systematic, however, and the contribution to
the total dispersion as a result of using the vector
subtraction technique is less than this value.
TABLE 5. SUMMARY OF INTERPRETED '•"Ar/Âr THERMOCHRONOLOGY.
MAGNETIZATION ACQUISITION AND STRUCTURAL EVENTS,
RED MOUNTAIN INTRUSIVE SYSTEM
Figure 12. Schematic block diagram of the
Red Mountain intrusive system showing relative ages of stocks and cooling of emplacement (see Table 5); all ages in millions of
years. Dashed lines represent approximate
contacts between intrusions. Stippled region
represents approximately the part of the system that cooled below -280 °C at 27.59 ±
0.03 Ma on the basis of six biotite 40Ar/39Ar
dates. Patterned region represents area of
•nagnetite-sericite alteration formed between
27.51 ± 0.03 and 26.95 ± 0.08 Ma at the end
of system cooling.
Age (Ma)
Pan of system
Geologic activity
(i.e., thermal event)
posi-26
Entire sysiem
Tilling.
ESE-side down
26.95 ± 0.08 10
27.51 3- 0.03
Seriate stock
Magnetite-seriate
alteration
27.59 * 0.03
Core of system
Cooled below 280 °C
>28.4 i 03
Ute stock
Emplacement
-28.71 ~ 0.08
Vasquez stock
Emplacement
--28.71 * 0.08
Seriate and
Henderson stocks
Em place men l
-•28.71 ± 0.08
Urad porphyr>
Emplacement
29 4 and 29.8
Lamprophyre and
rhyolne dikes
Emplacement in Silver Plume
Granite
> 29.85 ± 0.34.
possibly ;• 30.3X t 0.09
Porphyry of Red
Mountain and older
surface stocks
Emplacement in
Silver Plume Granite
Geological Society of America Bulletin, Augusl 1992
Effect of event
on magnetization
Normal polarity of Henderson Mine
stocks completely blocked
Reverse to normal polarity
1042
GEISSMAN AND OTHERS
Some dispersion of data from a single site or
stock must be attributed to processes affecting
volumes as small as individual specimens. Demagnetization of at least six specimens per sample permitted estimation of within-sample dispersion for several randomly chosen samples. As an
example, six specimens from sample 8C (site
HENS in the Seriate stock) give a mean direction with declination of 358° and inclination of
+57° (a95 = 12.9°, k = 27.8). Although betweenspecimen dispersion is usually less than that between samples (within site), implying sampleorientation errors, between-specimen dispersion
may be considerable. Magnetizations in the Red
Mountain stocks, each with a cross section on
the order of 0.04 to 0.09 km 2 at the elevation of
the Henderson Mine, were acquired over several
thousand years, if not considerably longer, during blocking below 580 °C and alteration. For
the surface rocks with abundant hematite, at
least part of the dispersion may arise from alteration, because Heider and Dunlop (1987)
showed that secondary magnetizations acquired
during the oxidation of magnetite to hematite
may have no record of either a pre-existing
magnetization or the ambient field attending
alteration.
Within-site dispersion of all Henderson stocks
may also reflect past secular variation, and this
dispersion may be compared with predicted angular variance values on the basis of models of
field variations. Although data from young lava
flows compare favorably with model predictions, we are not sure whether data from slowly
cooled intrusions can be compared with values
provided by such models. For a site latitude of
39.8°, McFadden and McElhinny's (1984) paleosecular variation model for Cenozoic virtual
geomagnetic pole (VGP) data predicts a VGP
angular dispersion of 16.3°. McFadden and others' (1988) model G, on the basis of data from
lavas <5 Ma in age, predicts a value of 16.1°.
Individual sites and individual stocks of the Red
Mountain intrusive system have angular dispersions that usually exceed 15°, suggesting that
magnetizations were acquired over extended
time periods.
has been tilted in an east-southeast-side-down
fashion.
Stocks of the intrusive system give magnetizations of northwest declination and moderate
positive inclination (and antipodes); they do not
resemble mid-Cenozoic reference directions.
The tight grouping of pluton mean directions,
which themselves are moderately dispersed, and
the presence of dual polarities suggest that the
late OJigocene field has been time averaged by
the ensemble of all stocks.
Grand mean directions for the intrusive system (Fig. 13) were determined in two ways
(Table 3). We first used mean directions from
the 10 stocks for which 0:95 values were <15°
(Tables 1 and 2). The mean directions from the
remaining three stocks are statistically identical
to the overall mean for the complex at a 95%probability level (McFadden and Lowes, 1981),
yet they are determined at a poor level of precision. Alternatively, we assumed that each site,
rather than each stock, provided an independent
record of an ambient field, resulting from a
complicated cooling and alteration history. All
site means that had a95 values of <20° (34 of 39
sites) were included. The mean determined by
this method is statistically indistinguishable, at a
99%-probability level (McFadden and Lowes,
1981), from that determined using stock means.
The pluton means yield a VGP angular variance
of 1.7°, whereas the site means yield a VGP
angular variance of 12.5°, values less than those
Deformation of the Red Mountain
Intrusive System
Figure 13. Partial equal-area projection of
in situ stock (10 stocks accepted) mean and
site (34 sites accepted) mean directions (solid
circles) and associated projected cones of 95%
confidence. Solid squares represent midOligocene expected directions determined
from paleomagnetic poles of Irving and
Irving (1982) (I+I) and Diehl and others
(1983) (D).
Wallace and others (1978) recognized the
asymmetry of the Henderson deposit and suggested that parts of the deposit might have been
deformed since stock emplacement and mineralization. Field evidence described by Geraghty
and others (1988) suggests that Red Mountain
1+182
for individual samples, sites, and stocks. The
12.5° variance is closer to that expected for a
small number of independent field observations.
We interpret the small angular-dispersion value
derived from stock means to imply an averaging
of the late Oligocene field by each stock.
Expected time-averaged geomagnetic field directions for central Colorado (Fig. 13) have
been calculated from paleomagnetic poles compiled by Irving and Irving (1982) (30 Ma mean)
and Diehl and others (1983, 1988) (38-22 Ma
results). In-situ magnetizations from the intrusive system are discordant in both declination
(westward) and inclination (shallower) from
either of the above reference directions. The discrepancy between observed and expected directions may be explained by rotation about one
structural axis or a combination of several axes
of differing orientations. Given the absence of
late Oligocene and younger strata near Red
Mountain, we evaluate the discrepancy in terms
of a single event that uniformly deformed the
entire Red Mountain area. This approach may
reveal that a particular bounding fault or set of
faults controlled structural adjustment. Deformation in all likelihood consisted of a more
complicated sequence of movements involving
differential adjustments on several bounding
faults.
Because the discrepancy in paleomagnetic inclination is less than that in declination, singlestep deformations are of two types. The first is
true counterclockwise rotation about nearvertical axes in response to shear along a single
fault or sets of faults bounding Red Mountain.
This mechanism is unlikely given the required
amount of rotation for all orientations of rotational axes that could be related to known structures. On both local and regional scales, there is
no evidence to suggest an amount of strike-slip
along faults in the Front Range of Colorado in
late Oligocene and younger time (Taylor, 1975;
Tweto, 1975; Theobald, 1965) large enough to
cause ~30° of vertical axis rotation (Mackenzie
and Jackson, 1983).
The second type of deformation—moderate
tilting about a near-horizontal axis—can account for the discrepancy. From 15° to 25° of
west-side-down tilting about an axis oriented
N15°E is required to "correct" the observed
data into agreement with expected directions.
This axis is perpendicular to the strike of the
West Fork of the Clear Creek fault and is within
10° and 30° of strikes of the Vasquez Pass and
Berthoud Pass fault zones (Figs. 1 and 2), respectively. Deformation involving dip-slip along
the West Fork of the Clear Creek fault (Fig. 14)
1043
Clear Creek fault /Vasquez
\
\
/
Pass
/
;;':?:: Porphyry o f : ;
Figure 14. Cross section A-A' (Fig. 1) showing geometry of Cenozoic fault displacement in the Red Mountain area. H = Henderson stock; P =
Primes stock. On the basis of crosscutting relations, the most recent fault displacement is progressively younger to the northwest from the
Woods Creek fault. We estimate a minimum of 830 m of Neogene dip-slip displacement for the Woods Creek fault, 390 m for Vasquez Pass
fault, and 1,100 m for Clear Creek fault Modified from Geraghty and others (1988).
may explain such tilting. With a hinge in the
middle of the Red Mountain block, about 540
m of dip-slip offset along the combined Woods
Creek and Berthoud Pass faults (southeast of
Red Mountain and the Vasquez Pass fault to the
northwest) (Fig. 14) would result in the inferred
tilt. This slip estimate is in accord with belt's
(1975) suggested upper limit of about 1 km for
Neogene displacement along normal faults in
north-central Colorado. We prefer a style of deformation involving variable dip-slip with offset
along at least two major structures to simple
dip-slip along either the Vasquez Pass or Berthoud Pass-Woods Creek faults. Dip-slip displacement along these northeast-trending structures and attending tilt would give a more
northward declination for the observed Red
Mountain mean direction.
Assuming that the data from the Red Mountain intrusive system record local tilting from
15° to 25° about an approximately NNE, horizontal axis, then the pre-tilt orientation of
mineralizing stocks can be determined. Plunges
of axes of the cylindrical Urad, Henderson, and
Seriate stocks define a great circle of attitude
N57°E, 66°NW. After restoring the great circle
to its pre-tilt position, the orientation is N52°E,
80°NW. This trend parallels the Woods Creek
strand of the Berthoud Pass fault zone, suggesting that structure controlled the emplacement of
the stocks. Correcting for east-side-down tilting
results in a more vertical orientation of stocks
and their contacts.
Implications of 40Ar/39Ar Age-Spectrum
Data for Intrusion Emplacement
and Acquisition of Magnetizations
The age-spectrum and magnetic polarity data
provide an improved assessment of the age of
intrusion, subsolidus cooling, alteration, and
structural history of the Red Mountain intrusive
system (Table 5), just as 40Ar/39Ar agespectrum data on individual mineral phases of
estimated isotopic closure temperatures have
been used to define the subsolidus tectonothermal history of intrusive igneous rocks and associated hydrothermal deposits and regionally
metamorphosed rocks in many settings (Sutler
and others, 1985; Snee and others, 1988; Foster
and others, 1990).
In the following discussion, we compare the
Red Mountain magnetic polarity and Âr/
39
AT isotopic age-spectrum data with published
geomagnetic polarity time scales. Considerable
controversy still exists about the accuracy of
these polarity time scale calibrations and some
calibrations are currently being revised substantially (W. A. Berggren, 1989, personal commun.). As a result, we are not able to confidently
correlate magnetization polarity in the intrusive
system with particular polarity chrons. The durations of individual magnetozones, however,
are far less controversial and have direct bearing
on the interpretation of the magnetization acquisition history of the Red Mountain intrusive
system.
Magnetization blocking spanned at least one
field reversal following intrusion of the reversely
polarized East Knob pluton and Rubble Rock
stocks, possibly during cooling of the porphyry
of Red Mountain. 40Ar/39Ar age-spectrum data
on orthoclase from this stock indicate cooling
below 350 °C possibly as early as 30.38 ± 0.09
Ma (Fig. 11A). Lamprophyre and rhyolite dikes
on the surface yield 40 Ar/ 39 Ar age-spectrum
data of 29.81 ± 0.10 (biotite) and 29.4 ± 0.9 Ma
(orthoclase). Although we were not able to obtain 40Ar/39Ar age-spectrum data for the reversely magnetized porphyry of the East Knob
pluton, the age spectrum data from the porphyry
of Red Mountain provide a basis for the interpretation that the polarity reversed to normal
before ca. 30 Ma. The time scale of Harland and
others (1982) gives the boundary between chron
9r and chron 9 at 29.94 Ma (Fig. 15). With the
exception of short polarity chrons, the immediately older and younger reverse to normal
transitions are placed at 31.2 and 28.3 Ma (Harland and others, 1982) and do not compare fa-
1044
GEISSMAN AND OTHERS
-23
23- •
tf>
|24-
-24 -
-25 -
-26
-27
JVj
EJ25-
si'5
13
o
0)
Q.
03
T3
0 26- •
O)
a]
to
I
CD
O)
03
<f
l)-
i
CO
O
fo
*-
CD
TJ
<27
I
-28
28- •
-29
29- •
-30
30 - -
-31
31 - •
-32
32- •
-33
33--
1
Figure 15. Geomagnetic polarity time scales for late Eocene and Oligocene time compared with zircon fission-track (F-T) dates, K/Ar isotopic
and 40Ar/39Ar age-spectrum determinations for the Red Mountain intrusive system. Numbers adjacent to fission track dates and isotopic age
determinations correspond to sample numbers in Table 4. Vertical lines indicate error bars; for the *°A.r/39\r age-spectrum data, O, B, and M
refer to orthoclase, biotite, and muscovite (sericite), respectively.
vorably with the Âr/Âr data. More recent
geomagnetic polarity time scales systematically
assign polarity chrons to younger ages during
this time period (Fig. 15) and would require
cooling of the porphyry of Red Mountain during
chron 11 (Montanari and others, 1988; Swisher
and Prothero, 1990; Mclntosh and others,
1992), possibilities which are not excluded by
the argon data. According to Mclntosh and others' (1992) internal calibration based on highprecision, sanidine Âr/^'Ar age-spectrum
data from volcanic rocks, the reverse to normal
transition during emplacement of the Red
Mountain surface rocks would be the 11R/11
chron boundary at ca. 29.5 Ma.
Rocks at the Henderson Mine levels cooled
later than those exposed at the surface. The
normal polarity porphyry of Urad Mine, the
deep-seated equivalent of the porphyry of Red
Mountain (Garten and others, 1988), yields disturbed orthoclase and biotite age spectra of 28.0
± 0.09 and 28.6 ± 0.3 Ma, respectively. The
Henderson, Seriate, and Primes stocks, which
intrude porphyry of the Urad Mine, also acquired characteristic magnetizations of normal
polarity. 40Ar/39Ar isotopic age determinations
from the Seriate, Vasquez, and Ute stocks indicate that orthoclase isotopic blocking occurred
between 28.5 and 28.7 Ma. Age-spectrum data
on biotite (average of 27.59 ± 0.03 Ma; Table 5;
Fig. 11C) specify when the central mineralized
zone cooled below -280 °C. Muscovite (sericite) determinations (26.95 ± 0.08 and 27.51 ±
0.03 Ma) are younger than those of biotite, be-
cause sericite formed after the system cooled
below biotite-blocking temperatures.
Whereas reverse polarity magnetizations of
low coercivity and (or) low-unblocking temperature spectra are present in some samples from
the subsurface rocks, we interpret the absence of
reverse polarity magnetizations of high coercivity and (or) unblocking temperature to suggest
that magnetization blocking occurred during
one normal polarity chron, after intrusion of the
Urad and Red Mountain stocks. Unblockingtemperature data support the likelihood that
blocking occurred at temperatures from much
higher than 500 °C to as low as 300 °C. The
40
Ar/39Ar orthoclase and biotite age spectra
give estimates for a lower bound of magnetization blocking. The time between 28.5 and 28.7
Ma is principally one of reversed polarity according to the time scales of Harland and others
(1982) (chron 8R), Montanari and others
(1988) (chron 9R), Harland and others (1989)
(chron 9R), Mclntosh and others (1992) (chron
10R), and Swisher and Prothero (1990) (chron
10R), and of normal polarity (chron 9) according to Berggren and others (1985). Although the
Berggren and others' (1985) time scale seems to
match the orthoclase age-spectrum and polarity
data more appropriately, we note that reverse
polarity chron 8R lies between the time of orthoclase and biotite closure in this time scale in the
Henderson Mine rocks. The position of normal
polarity chron 8 in the time scales of Harland
and others (1982) and Berggren and others
(1985), chron 9 in the time scales of Montanari
and others (1988) and Harland and others
(1989), and chron 10 in the time scales of Mclntosh and others (1992) and Swisher and
Prothero (1990) is consistent with the dates for
biotite closure. The general absence of reverse
polarity magnetizations in the Henderson Mine
stocks can be explained by emplacement before
about 28.5 Ma (Table 5). The entire system
cooled rapidly and magnetizations blocked to
temperatures below 280 °C during a single,
normal polarity chron, or, less likely, a time before 27.6 Ma dominated by normal polarity.
Chron 8, of ~0.8 m.y. (Harland and others,
1982) or 0.7 m.y. (Berggren and others, 1985)
duration; chron 9, of 0.7 m.y. (Montanari and
others, 1988) to 1.0 m.y. duration (Harland and
others, 1989); or chron 10, —0.6 m.y. duration
(Mclntosh and others, 1992; Swisher and
Prothero, 1990) may be the likely candidates for
the intervals of normal polarity remanence acquisition. Conclusions on the cooling and
magnetization-acquisition history for the Henderson Mine rocks cannot be applied to the
Vasquez and Ute stocks; these stocks were not
sampled for paleomagnetism. Similarities in
orthoclase and biotite cooling ages for these and
sampled stocks lead us to predict that the Vasquez and Ute stocks also have normal polarity
magnetizations.
CONCLUSIONS AND REGIONAL
TECTONIC IMPLICATIONS
Paleomagnetic and 40 Ar/ 39 Ar age-spectrum
data from most stocks of the Red Mountain intrusive system provide an improved understanding of the structural and cooling history of the
suite of intrusions host to a major porphyry
molybdenum deposit. Characteristic magnetizations, acquired during reverse polarity before ca.
30 Ma and normal polarity between 28.7 and
27.6 Ma, are discordant with expected directions for late Oligocene time. The discrepancy
implies about 15°-25° of east-side-down tilting
of the Red Mountain area about a northnortheast-trending axis and is consistent with
observed field relations.
The 40Ar/39Ar age-spectrum data are interpreted to indicate emplacement of surface stocks
of Red Mountain before 29.85 ± 0.34 Ma (possibly before 30.38 ± 0.09 Ma). Intrusion of the
Urad porphyry and the Seriate, Henderson,
Vasquez, and Ute stocks, all exposed in the subsurface, occurred between ca. 28.4 and 28.7 Ma.
Biotite age-spectrum data tightly define cooling
of the core of the mineralized system below
-280 °C at about 27.6 Ma. Magnetite-sericite
alteration, in particular of the Seriate stock, occurred between ca. 26.9 and 27.6 Ma. Magnetizations carried by maghemite in some of the
underground sites may have been acquired over
this time period and appear to have accurately
recorded the ambient field. Comparison of the
Red Mountain age-spectrum and polarity data
with published geomagnetic polarity time scales
fails to identify unequivocally chrons in which
the observed magnetizations were acquired. The
likely candidates for intervals of normal polarity
remanence acquisition are either Chrons 9 or 10.
Styles of local, basement-involved deformation similar to that affecting the Red Mountain
area have been described by Kellogg (1973) and
Hoblitt and Larson (1975) for the easternmost,
east-tilted flank of the Front Range. The absence
of layered rocks near Red Mountain precludes
directly relating tilting of area with post-middle
Oligocene tectonic events in much of central
Colorado and New Mexico. Early uplift and deformation along the margins of Precambriancored blocks in the southern Rocky Mountains
of Colorado and New Mexico have been ascribed to Laramide events of latest Cretaceous to
earliest Cenozoic age (Eaton, 1986; Oppenheimer and Geissman, 1989). Our interpretation of Red Mountain data indicates that blocks
within the uplifted ranges were tilted at least
locally in late Oligocene and younger time.
Other Precambrian-cored uplifts of the southern
Rocky Mountains may have been similarly deformed; the Red Mountain data should not be
interpreted to indicate uniform magnitude and
sense tilting of fault blocks in this part of the
Front Range. In the Jamestown area (Fig. 1),
about 15 km west of the eastern edge of the
Front Range, Sheldon and Geissman (1983)
concluded that early to mid-Tertiary stocks have
been tilted only slightly during uplift. Evidence
for regional, east-side-down tilting of the entire
Front Range would be manifested by differential
1045
structural relief, systematic changes in metamorphic facies of Precambrian rocks, and variations
in uplift ages of rocks at a constant topographic
level. Variations in uplift ages were not documented by Bryant and Naeser (1980) in their
fission track study in the Front Range of
Colorado. Deformation in other Precambrian
basement cored uplifts may be similarly complicated. Steidtman and others (1989) demonstrated late Oligocene to mid-Miocene reactivation and tilting of parts of the core of the
southern Wind River Mountains. Although tilting of the Red Mountain area may be in response to extension along the northern terminus
of the Rio Grande Rift, the absence of layered
strata precludes relating the timing of deformation to early (late Oligocene-early Miocene) or
late (late Miocene and Pliocene) phases of
northern Rio Grande Rift extension (Morgan
and Golombeck, 1984). Elsewhere in northern
Colorado, there is geologic evidence for differential uplift and block faulting in Miocene and
younger time (Izett, 1975; Tweto, 1980; Larson
and others, 1975).
ACKNOWLEDGMENTS
Financial and logistical support from the
Climax Molybdenum Company are most appreciated. Financial support was also provided
by Sigma Xi to G. W. Graaskamp. Mark Hudson and Ken Shonk aided in field sampling, and
Mark Hudson also measured Curie temperatures
at the U.S. Geological Survey, Denver. Comments by G. Brent Dalrymple, Steve Harlan,
Richard Reynolds, and Deke Schnaebel improved the manuscript. We thank the Bulletin
reviewers, Mike McWilliams and an anonymous colleague; Associate Editor Bob Butler; and
Editor Art Sylvester for their comments. Discussions with W. A. Berggren about problems with
current geomagnetic polarity time scales were
enlightening. Dag Lopez prepared some of the
illustrations.
APPENDIX: PALEOMAGNETIC AND
40
Ar/ 39 Ar AGE-SPECTRUM METHODS
At 39 sites in the intrusive system (Figs. 2 and 3),
215 samples were collected for paleomagnetic study.
There were 60 samples collected from 5 intrusions at 9
sites on the surface; 155 samples were collected from 8
stocks exposed in underground workings of the
Henderson Mine. The underground sites are distributed over 180 vertical meters on the 81,00-, 8,050-,
7,755-, 7,625-, and 7,500-ft levels (Fig. 3).
All samples were collected as independently oriented blocks using a magnetic compass. The intensity
of NRM of most samples is <0.1 A/m. Samples on
the surface of Red Mountain were collected from the
1046
base of the least prominent outcrops to avoid the effects of lightning strikes on the rocks. During underground sampling, we took azimuthal backsights to
avoid effects of anomalous inductions due to ferromagnetic construction and mining equipment. Because
of the intensely fractured nature of the rocks, we could
not always collect seven or more samples per site.
Sites were chosen with two principal goals: (1) to
sample as many separate intrusions as possible and
(2) to minimize the possibility of complete thermal
resetting of TRM's and isotopic ages in older stocks by
progressively younger stocks. An Oligocene field direction is best averaged by sampling many small, rapidly cooled plutons, each of which might provide a
record of the field during a geomagnetically short period of time. We attempted to selectively sample perimeters of stocks most distant from contacts with
younger stocks rather than interiors. In some places,
sites in older stocks were intentionally sampled close
to younger stocks for "contact" tests.
Block samples were drilled in the laboratory to obtain at least three standard (11 cm3) cylindrical specimens and were measured using a computer-interfaced
Schonstedt SSM-1A spinner magnetometer. At least
one specimen per sample was subjected to progressive
AF demagnetization using a Schonstedt GSD-1 singleaxis system. For information on the distribution of
laboratory unblocking temperatures (T|ub's) of magnetizations, specimens from at least two samples per
site were subjected to progressive thermal demagnetization using a Schonstedt TSD-1 furnace. In some
samples, especially for those from surface rocks, AF
demagnetization to 100 mT was not enough to remove >50% of the NRM (typically, much more than
5% of the NRM remained). Continued thermal treatment was used to more completely isolate the remanence. In general, demagnetization yielded consistent
behavior characterized by linear segments, and one or
more of the magnetization components could be identified using orthogonal demagnetization diagrams
(Zijderveld, 1967). Directions, relative intensities, and
general coercivity/T|ub spectra of all components were
determined using demagnetization diagrams, plots of
normalized intensity versus demagnetization, and vector subtraction (Hoffman and Day, 1978) methods. As
a check on the results of vector subtraction, principal
component analysis (Kirschvink, 1980) was applied to
the demagnetization data from about 5% of the
samples.
Rock magnetic tests were made on representative
samples from all intrusions and consist of acquisition
and backfield demagnetization of IRM, determination
of Curie temperatures, and measurement of low field
susceptibility after each thermal treatment. An electromagnet capable of 1.2 T inductions supplied DC
inductions for IRM tests. Curie temperatures were
measured using both a vertical balance (at the U.S.
Geol. Survey laboratory, Denver) and a horizontal
balance (at the Univ. of New Mexico laboratory), in
saturating inductions of -0.2 T. Samples of all intrusions were inspected with reflected light petrography
to identify parageneses of visible magnetic phases.
High-precision 40Ar/39Ar age-spectrum data were
obtained to define the thermal history of the intrusive
system with the principal intent of quantifying the
number and duration of mineralizing events. From
stratigraphically known sections of drill core, 21 muscovite, biotite, and potassium feldspar samples used for
argon thermochronology were carefully selected. Pure
GEISSMAN AND OTHERS
mineral separates could be extracted directly from
some cores; from other cores, standard separation
techniques were used to obtain pure samples. X-ray
diffraction patterns of all potassium feldspars showed
them to be orthoclase. All mineral separates were unaltered except one potassium feldspar from the Seriate
stock that included -10% illite formed during a later
(the magnetite-sericite) alteration event.
Samples for argon analysis were irradiated in two
separate groups: one in the U.S. Geological Survey
TRIGA reactor and one in the University of Michigan
Phoenix reactor, using normal encapsulation procedures described in Snee and others (1988). The isotopic composition of argon was measured at the U.S.
Geological Survey, Denver, Colorado, using a Mass
Analyzer Products, Limited, 215 series, rare-gas mass
spectrometer. (Trade, product, or firm names are used
for descriptive purposes only and do not imply an
endorsement by the United States government.) Isotopic abundances were corrected for mass discrimination. The neutron flux monitor used in this study is
hornblende MMhb-1, the age of which is 520.4 Ma
(Alexander and others, 1978; Samson and Alexander,
1987); an error of 0.25% (1 sigma) was determined
experimentally by calculating the reproducibility of
several aliquants of argon for all monitors. Samples
irradiated at the University of Michigan were corrected for irradiation-produced, interfering isotopes of
argon by measuring production ratios for those isotopes in pure ^SC^ and CaF2 irradiated simultaneously with the samples. For samples irradiated at the
U.S. Geological Survey, production ratios determined
for this reactor by Dalrymple and others (1981) were
used; the assumptions made in their study have since
been shown to be adequate based on production ratios
determined for many irradiations. Corrections were
made for additional interfering isotopes of argon produced from irradiation
of chlorine (Roddick, 1983).
Quantities of 39Ar and 37Ar were corrected for radioactive decay. Constants used in age calculations are
those of Steiger and Ja'ger (1977). Error estimates for
apparent ages of individual temperature steps were
assigned by using the equations of Dalrymple and others (1981). The equations were modified to allow the
option of choosing the larger of two separately derived
errors in the 40 Ar R / 39 Ar K ratio, the calculated error
from differential equations or the experimentally determined error derived from the reproducibility of
analyses of multiple aliquants of argon from the
monitors. Dates are reported with 1-sigma errors. Age
plateaus were determined by comparing contiguous
gas fractions using the critical value test described by
Dalrymple and Lanphere (1969).
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Deformation and age of the Red Mountain intrusive system (Urad

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